1. Field of the Invention
The present invention relates to a magnetic recording medium. More particularly, the invention relates to an improvement of a magnetic recording medium comprising a nonmagnetic support and a magnetic recording layer.
2. Description of Prior Arts
A magnetic recording medium basically comprises a nonmagnetic support and a magnetic recording layer provided on the support, the magnetic recording layer comprising a ferromagnetic powder dispersed in a binder.
At present, magnetic recording media are used in a variety of forms, for example, an audio-tape, a video-tape, a floppy disk and so on. Naturally, magnetic recording media are required to have various characteristics according to their forms and purposes.
The audio-tape such as an audio cassette tape is frequently used for recording music. In this case, a magnetic recording medium is required to show excellent electromagnetic conversion characteristics such as satisfactory frequency characteristics and a superb reproducibility of original sounds. Also, a tape should have an improved running property, durability and heat resistance so as to meet the outstanding needs since the magnetic tape-recording medium is now generally employed in automobiles, a cassette tape-recorder with radio and a small-sized tape player.
In the case of a video cassette tape, highly dense recording is now possible by shortening recording wavelengths or narrowing a track width. A magnetic recording medium is, therefore, required to show excellent electromagnetic conversion characteristics such as high video outputs, high S/N ratio and excellent reproducibility of originally recorded pictures. Further, the recent wide spread use of portable VTRs request a tape to have an improved running property and durability. While tapes need to be highly durable, the tape thickness tends to be thinner, for example, less than 20 .mu.m.
Thus, a magnetic recording medium in the form of tape such as a video-tape or an audio-tape should run in contact with a fixed magnetic head, a drum, guide drums and so on. Therefore, a magnetic recording medium needs to have not only excellent electromagnetic conversion characteristics but also a low friction coefficient which enables smooth and stable running for a long period of time. Further, the tape needs to have satisfactory durability and heat resistance, because it should run under a great amount of friction with a magnetic head.
There have been several proposals to fulfill the above-mentioned requirements for the characteristics of a magnetic recording medium.
For example, it is reported that the running property of a tape is improved by incorporating into the magnetic recording layer a higher fatty acid, a higher fatty acid ester, a paraffinic hydrocarbon or a silicone oil. Thus, lubricity is imparted to the tape to prevent wear of the tape caused by the friction with the tape head, thereby improving not only the tape durability but also the magnetic conversion characteristics of a magnetic recording medium. These proposals are seen in the specifications of Japanese Patent Publications No. 41-18064, No. 43-186, No. 43-669, No. 47-15624, No. 56-20611 and No. 56-26890 and U.S. Pat. Nos. 2,654,681, 3,470,021, 3,497,411, 3,525,694, 3,634,253, 4,110,503, and 4,135,016.
The running property of a tape can be improved to a certain extent by incorporating a higher fatty acid, a higher fatty acid ester, a paraffinic hydrocarbon or a silicone oil into a magnetic recording layer. By these methods, however, the characteristics required for a magnetic recording medium, especially magnetic conversion characteristics, are hardly improved. There is also a problem that a magnetic recording medium using a fatty acid shows poor heat resistance and shrinks at elevated temperatures.
When a higher fatty acid, a higher fatty acid ester, a paraffinic hydrocarbon or a silicone oil is included in a magnetic recording layer, the phenomenon called "blooming" (the above compound oozes out from the surface of a magnetic recording layer) may sometimes take place. This blooming phenomenon makes a tape sticky and consequently reduces the running smoothness. The blooming phenomenon is especially observed in the case that a silicone oil is used. Also, a tape containing the above compound may exhibit, on the contrary to the original purpose, high coefficient of kinetic friction with a head drum under heavy load. In such instance, the abnormal sounds called "creaks" are sometimes generated by the friction between the tape and the drum. Further, the compounds oozed out through blooming are very likely to catch dusts, whereby the magnetic recording medium is contaminated.